1. Field of the Invention
The present invention relates to a toner for developing electrostatic images used for electrophotography, electrostatic recording, electrostatic printing and other purposes, and a charge control agent capable of controlling the amount of charges of the toner.
2. Description of the Prior Art
In copying machines and other apparatuses based on electrophotography, various dry or wet toners containing a coloring agent, a fixing resin and other substances are used to visualize (develop) the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an inorganic or organic photoconductive substance.
The chargeability of such toners is the most important factor in electrostatic latent image developing systems. Thus, to appropriately control the charge amount of toner, a charge control agent providing a positive or negative charge is often added to the toner.
Examples of charge control agents providing a positive charge for toner in actual application include the nigrosine dyes disclosed in Japanese Patent Provisional Publication No. 2427/1966 and other publications. Examples of charge control agents providing a negative charge for toner include the metal complex dyes described in Japanese Patent Examined Publication Nos. 20153/1966, 17955/1968 and 26478/1970 and other publications.
However, most of the above-mentioned charge control agents have a complex structure and their stability is low. For example, they are liable to decompose or deteriorate to lose their charge control performance due to mechanical friction and impact, changes in temperature or humidity, electric shocks, light irradiation and other causes. In addition, most of them have been relatively densely colored, thus lacking versatility for use in color toners.
In recent years, various charge control agents free of such problems have been disclosed, including the metal complexes of salicylic acid or derivatives thereof disclosed in U.S. Pat. Nos. 4,206,064 and 4,656,112, and the metal complexes of aromatic oxyaldehydes disclosed in Japanese Patent Provisional Publication No. 206769/1988.
However, these charge control agents are insufficient with respect to resin affinity, dispersibility, etc., and in addition, they comprise a compound containing a heavy metal such as chromium, cobalt, copper or zinc; most of them involve a problem in safety or hygiene to the human body.
On the other hand, there are known toners supplemented with various phenol compounds for the purpose of improving the fixability and offset property of the toner or preventing the deterioration of a surface-treated carrier. For example, Japanese Patent Provisional Publication No. 138357/1988 discloses a toner containing an oligomer of a phenol compound having an alkyl-substituted amino group. Japanese Patent Provisional Publication No. 291569/1990 discloses a toner incorporating a coloring agent resulting from treatment of a xanthene dye with a compound having a phenolic OH group. Japanese Patent Provisional Publication No. 266462/1988 discloses toners containing a compound such as
2,6-di-tertiary-butyl-p-cresol, PA1 2,6-di-tertiary-butyl-4-ethylphenol, PA1 2,2'-methylene-bis-(4-methyl-6-tertiary-butylphenol) or PA1 2,2'-methylene-bis-(4-ethyl-6-tertiary-butylphenol). PA1 n represents an integer of 1 to 8.
However, the developing agents incorporating such a toner do not have sufficient chargeability, though they are excellent in fixability, offset resistance and durability.
The object of the present invention, developed in view of the above problems involved in conventional methods, is to provide a charge control agent which is good in heat resistance, pulverizability and resin affinity and dispersibility, excellent in charge control property stability to changes in temperature and humidity, i.e., environmental resistance, and stability of charge control property over time, i.e., storage stability, versatile for use in color toners, including the three subtractive primaries yellow, magenta and cyan colors, and safe to the human body, and a toner for developing electrostatic images which is excellent in environmental resistance and storage stability with respect to chargeability and can be used as a toner with various chromatic or achromatic colors.
Although one of the present inventors has already developed the invention disclosed in Japanese Patent Provisional Publication No. 201378/1990, the present invention is directed to providing a charge control agent offering more rapidly rising charging and a toner of developing electrostatic images permitting a more rapid rise of charging.